FIG. 4 is a circuit diagram of an example of conventional driver circuits, which shows a configuration of a source type driver circuit, in particular. In the following description, the reference symbol M denotes a field effect transistor and Q denotes a bipolar transistor. As shown in FIG. 4, a high potential supply voltage Vcc (e.g., 5 V) is supplied to a supply voltage terminal VCC of an integrated circuit IC to drive a plurality of external circuits through driver output terminals D1, D2, D3, . . . , respectively. Here, for instance, the driver output terminal D3 is connected to the base of a PNP type bipolar transistor Q14 whose collector is connected to an output terminal Out4. The emitter of this transistor Q14 is connected to the high potential supply voltage Vcc and the base thereof is connected to the same high potential supply voltage Vcc through a resistor R14. Further, a load resistance RL is connected as a load to the output terminal Out4 connected to the collector of the transistor Q14. Further, the transistor Q14, the resistor R14 and the output terminal Out4 are all discrete parts connected as an external circuit of the integrated circuit IC.
The operation of the circuit as shown in FIG. 4 will be described hereinbelow. The integrated circuit IC is a circuit formed by integrating a plurality of functional elements, which executes predetermined various functions internally on the basis of the high potential supply voltage Vcc supplied to the supply voltage terminal VCC. When the integrated circuit IC outputs drive signals (e.g., displaying, printing, driving, etc.) to the outside, these drive signals are outputted through the driver output terminals D1, D2, D3, . . . , respectively.
In this case, if a driven element connected to the driver output terminal D3 of the integrated circuit IC is a load resistance RL, for instance and further the driving capability of the driver output terminal D3 is enough to drive the load resistance RL, it is possible to directly connect the load resistance RL to the output terminal D3 in such a way that a required driving current can be supplied directly from the integrated circuit IC to the load resistance RL.
In practice, however, there exist some cases where the driving capability of the driver output terminal D of the integrated circuit IC is not enough to drive the load resistance RL directly due to the microminiaturization of the integrated circuit IC. In this case, it is necessary to connect an additional driver circuit composed of discrete parts externally to the integrated circuit IC.
That is, the transistor Q14 is connected to the driver output terminal D3 for the purpose as described above. Here, when the driver output terminal D3 is at a high level, the base of the transistor Q14 is held at the high potential supply voltage Vcc through the resistor R14, so that the transistor Q14 is kept turned off. As a result, no current is supplied from the output terminal Out4 to the load resistance RL. In contrast with this, when the driver output terminal D3 changes to a low level, a base current flows from the emitter to the base of the transistor Q14 toward the driver output terminal D3, so that the transistor Q14 is turned on. As a result, a load current flows from the high potential supply voltage Vcc to the load resistance RL through the transistor Q14 and the output terminal Out4. In other words, as far as the output terminal D3 of the integrated circuit IC is provided with a base current absorbing capability sufficient to turn on the transistor Q14, it is possible to drive the load resistance RL.
As described above, in the circuit as described above, the capability of each of the driver output terminals D1, D2 and D3 of the integrated circuit IC is such an extent as to activate each of the external PNP transistors. In other words, since it is unnecessary for the integrated circuit IC to drive other circuits directly, a high breakdown voltage is not required for the integrated circuit. On the other hand, since the PNP transistor connected externally is a discrete part, there exists no problem with respect to the breakdown voltage of the transistor. Accordingly, this method of increasing the driving capability of the integrated circuit IC by connecting discrete parts externally thereto has been so far widely adopted to improve the driving capability of the integrated circuits.
On the other hand, however, since the PNP transistor is low in current amplification factor h.sub.fe, it is necessary to supply a sufficiently large base current to the transistor in order to obtain a large driving current. However, when a large driving current is supplied from the output terminal Out4 to the load resistance RL, since the current flowing through the driver output terminals D1, D2, D3, . . . of the integrated circuit IC also increases, there exists a problem in that the power consumption of the integrated circuit IC inevitably increases. In addition, since discrete parts must be connected externally, there exists another problem in that the circuit configuration is complicated externally and the size thereof increases.
To overcome these problems, a source type driver circuit is well known such that the load can be directly driven by turning on or off a transistor provided in the integrated circuit itself. FIG. 5 shows an example of the conventional driver circuits as described above. In FIG. 5, two N-channel MOS transistors M15 and M 25 are connected in series to each other. The source of the transistor M25 is grounded. The drain of the transistor M15 is connected to a supply voltage VDD via a resistance R15 and further to the gate of a P-channel transistor M35. The source of the transistor M35 is connected to the supply voltage VDD, and the drain thereof is connected to an output terminal Out. A load resistance RL is connected to the output terminal Out as a load.
The circuit shown in FIG. 5 is integrated together with another integrated circuit. The gate of the transistor M35 is controlled by controlling the gates of the series-connected transistors M15 and M25 on the basis of the internal operation of the integrated circuit. That is, when the two transistors M15 and M25 are kept turned off, since the supply voltage VDD is applied to the gate of the transistor M35 through the resistance R15, the transistor M35 is turned off, so that no current flows through the load resistance RL connected to the output terminal Out. On the other hand, when the two transistors M15 and M25 are turned on, since the gate of the transistor M35 is set to the ground level in potential through the resistance R15, the transistor M35 is turned on, so that a drive current flows from the supply voltage VDD to the load resistance RL through the transistor M35 and the output terminal Out.
In the above-mentioned driver circuit constructed as shown in FIG. 5, since an external driver circuit composed of discrete parts is not connected to the integrated circuit, the load resistance RL can be driven directly by the transistor M35 formed in the integrated circuit, with the result that there exists such an advantage that the circuit configuration can be simplified. On the other hand, however, since a higher voltage (an addition of the supply voltage VDD and another voltage (e.g., supply voltage) of the integrated circuit) is applied to the transistor M35 when the transistor M35 is kept turned off, there exists a problem in that the transistor M35 must be high in breakdown voltage.